That's the downside of E5600-series, it only supports DDR3-1066 natively. You need to find an X5650 or something similar, with DDR3-1333 support. Then you can safely OC your RAM to its fullest potential, while having an option of high and moderate overclock on your CPU.
I was pushing 1600+ on the E5640s, they should be able to handle that fine. 1866 might be a bit trickier but I know it can be done as well.